1. Field of the Invention
The present invention relates to a wafer heating apparatus to be employed for heating a semiconductor wafer homogeneously, and especially relates to a ceramic heater to be employed for a wafer heating apparatus.
The present invention relates to a method for the wafer heating apparatus and the ceramic heater for heating a wafer homogeneously.
2. Description of the Prior Art
A wafer heating apparatus has been used for, for example, heating a semiconductor wafer, forming a semiconductor thin film on a wafer to be used for a liquid crystal substrate or a circuit substrate, and forming a resist film on a wafer by drying and baking a resist liquid applied to the wafer.
The wafer heating apparatus has been used for, for example, heating a semiconductor wafer (hereafter simply referred as to a wafer) at the time of film formation treatment of a semiconductor thin film, etching and baking treatments for a resist film in the semiconductor device fabrication process.
A batch type apparatus for collectively carrying out film formation treatment for a plurality of wafers has conventionally been employed for semiconductor device fabrication apparatus. Recently, as wafers have been enlarged, e.g. into 8 to 12 inches in diameter, a technique, referred to as one-by-one treatment, has been employed in order to heighten the treatment precision. However, by the one-by-one treatment, the number of wafers to be treated for one time is decreased, so that the time taken to treat the wafers is required to be shortened. Consequently, it is required for a wafer supporting member to shorten the heating duration of a wafer, quickly perform attachment of the wafer thereto and detachment therefrom, and therefore improve the heating temperature precision as well.
As shown in FIG. 14, there has been employed a conventional wafer heating apparatus 31 including a heating plate 32 which is a metal plate made of aluminum or stainless steel and its upper face being used as a mounting surface 33 to mount a wafer thereon, a plurality of sheathed heaters 25 attached as heating element, to the lower face of the heating plate 32, and a presser plate 24 for fixing the heaters 25. Such a heating apparatus 31 is integrated with a container 27 containing the sheathed heaters 25 and lead wires 26 are connected to the sheathed heaters 25.
Such a heating apparatus 31 has been employed for drying a liquid resist coated on a semiconductor wafer W by mounting on the mounting surface 33 and heating the semiconductor wafer W by the heaters 25.
The heating plate 32 made of a metal has been formed rather thick to minimize deformation which would be made in repeatedly heating and cooling processes, and therefore the heating plate 32 has disadvantageously had high thermal capacity, and required considerably long time periods to heat and cool the heating plate effectively, resulting in low productivity.
There has been employed, as another wafer heating apparatus, as shown in FIG. 15, a ceramic heater 31 comprising a ceramic heating plate 32 in the form of a disk, which is made of alumina, silicon nitride, or aluminum nitride, and a heating element 35 buried within the heating plate 32. In this example, the heating apparatus 31 utilizes the upper face of the heating plate 32 as a mounting surface 33 and further includes terminals 36, for power supply, in the lower face, which terminals 36 are connected to the heating element 35 buried within the ceramic body. Although such a type heating apparatus has an advantage to be make a heating plate 32 thinner, it has been impossible to trim the heating resister for adjusting uniform desired resistance distribution because of the buried heating element. Consequently, this type of apparatuses have been not suitable to homogenize temperature distribution over the overall mounting surface 33.
Further, as another heating apparatus 31 for drying a resist film on a semiconductor wafer, an apparatus 31 is disclosed, for example, in Japanese Patent Publication No. 11-2837729 in which this conventional wafer heating apparatus, as illustrated in FIG. 11, is composed of a supporting body 41 on a shallow container, a heating plate 32 of a nitride or carbide ceramic provided on the supporting body 41, and a plate-like reflector 43 interposed immediate under the heating plate 32 over the bottom 41a of the supporting body 41.
In FIG. 12, power supply terminals 37 are joined by soldering to terminal parts of heating element 35 for even heating and the power supply terminals 37 are inserted into the through holes 67 formed in the plate-like reflector 43. Some holes 46 for leading out lead wires are formed in the outer circumference of the bottom part 41a. Lead wires 26 are inserted into the holes 46 to supply an electric current to the heating element and connected to the power supply terminals 37.
The heating apparatus of FIG. 12 is for drying a silicon wafer coated with a photosensitive resin at a high temperature (500xc2x0 C. or higher) using the heating element in the heating plate 32 supported by dummy pins 47 in an opening 46 of the supporting body 41.
The heating plate 32 with the heating element therein is designed to in the circular form and have approximately same diameter as that of the opening part 44 of the supporting body 41. As illustrated in FIG. 12, the heating plate 32 is of multilayer structure, in which the heating element 35 are buried in between the respective ceramic layers. That is, the heating element 35 of this apparatus are not at all exposed to the outer surface of the heating plate 32. The power supply terminals 37 relevant to electric power input to the heating element 35 are jointed to the heating element 35 by soldering.
The supporting body 41 is a aluminum made member having a bottom and has the opening part 44 with a circular cross-sectional on the upper side. Three holes 45 for inserting wafer-supporting pins therein are formed in the center area of the supporting body 41. By raising and lowering the upper ends of the wafer-supporting pins while supporting a wafer W thereon, the wafer W can easily be transformed to and received from a transporting apparatus, respectively.
Japanese Patent Publication No. 11-40330 discloses a heater for drying a photosensitive resin formed on the wafer surface. The structure of the heater, as referred to FIG. 13, includes a heating plate 32 made of a ceramic and a heating element 35 formed on the lower surface of the heating plate 32, wherein a metal sintered body 34 produced by sintering particles of one or more metal particles selected from gold (Au), silver (Ag), platinum (Pt), palladium (Pd), lead (Pb), tungsten (W), and nickel (Ni) is formed on the ceramic surface, and another metal coated layer 34 is provided on the metal sintered body 33, the metal coated layer 34 being composed of one or more metals selected from Au, Ag, Pd, Pt, and Ni.
In the above case, power supply terminals 37 are fixed in the heating element 35 by soldering. A wafer W can be set spaced from the heating plate 32 by lift pins 39.
An aluminum nitride-based ceramic and silicon carbide-based ceramic, having high thermal conductivity, are both suitable for the heating plate 32 of a semiconductor wafer heating apparatus required for carrying out homogeneously heating the wafer. Nevertheless, silicon carbide-based ceramics can more preferably be used for the heating plate 32 to be employed for the process of drying photosensitive resin films formed on wafers. That is because the aluminum nitride-based ceramic reacts to moisture in air at high temperature to generate ammonia gas which adversely affects the photosensitive resin. As a result, silicon carbide ceramics are more useful for heating plates 32 to be employed for process of drying of the photosensitive resin.
Although Silicon carbide is suitable for ceramic material composing a heating plate 32 from the point of view of high thermal conductivity, this material has semiconductive property to cause patterned resistors of the heating element to conduct electricity, and also electrodes connecting these resistors to conduct electrically, via the ceramic material, particularly, in the case of forming the heating element in the inside and on the surface of the ceramic.
Deformation, especially warp, or bent, of heating plates 32 has often been caused after sintering. Especially, at each time the gap between the heating plate 32 and a wafer W is changed over the area of the surface, the temperature distribution set on the surface of the heating plate 32 must be reset or the heating of positions of the wafer surface near any widened gap between the wafer and the mounting deformed surface of the heater would be delayed during the temperature increasing after a new wafer is substituted, resulting in a non-uniform temperature gradient on the wafer surface.
Further, as for a substrate of the heating plate of silicon carbide-based ceramic, it has been difficult to form a stable insulating layer between the silicon carbide ceramic and the heating element.
At the time of forming heating element, there occurs problems that the dispersion of the thickness of the heating element printed on an insulating layer is widened if the flatness of the insulating film is inferior and the temperature distribution therefore becomes uneven in the heating plate to result in unstableness of the temperature of a wafer.
An object of the present invention is to provide a wafer heating apparatus including a heat-homogenizing plate which can heat a semiconductive wafer quickly with a homogeneous temperature over the surface of the wafer and a ceramic heater for using such a wafer heating apparatus.
Another object of the invention is to provided a heat-homogenizing plate capable of preventing deformation during quick heating, to heat a wafer with even surface temperature distribution of the wafer.
Another object of the present invention is to provide a heat-homogenizing plate having a high thermal conductivity and being effectively capable of electrically insulating a heating element provided on the underside of the heat-homogenizing plate.
Another object of the present invention is to provide a method for the wafer heating apparatus and the ceramic heater for heating a wafer quickly and homogeneously.
A wafer heating apparatus of the invention comprises a heat-homogenizing plate made of a ceramic substrate and having one major surface for employing as a mounting surface for a wafer, a heating element which is provided to the heat-homogenizing plate, and pad electrodes fixed on the heating element to be electrically connected to the heating element, wherein the mounting surface of the heat-homogenizing plate is made slightly convex to provide a proper gap between the mounting face and the underside of the wafer to be evenly heated. Consequently, the evenly heating property of the heat-homogenizing plate to heat a wafer is improved, to homogenize the temperature distribution over the allover area of the wafer. The heating element preferably may be applied on said other major surface of the heat-homogenizing plate to facilitate the adjusting of the resistance distribution of patterned thin resistors making up the heating element.
The present invention further includes a wafer heating apparatus comprising a heat-homogenizing plate made of a ceramic substrate sintered of silicon carbide or boron carbide and having one major surface for employing as a mounting surface for a wafer, an insulating layer formed on said other major surface of the ceramic substrate, and a heating element formed on the insulating layer. The insulating layer can insulate the heating element from the semiconductive ceramic substrate. Such an insulating layer preferably may comprise an insulating glass or a heat-resistant insulating resin.
The wafer heating apparatus of this structure permits the heat-homogenizing plate to be sufficiently thin that the apparatus exhibits rapidly rising and enhanced homogenizing temperature property of the heat-homogenizing plate for heating a wafer. Especially, the heat-homogenizing plate may have a thickness of 2 to 7 mm, to assure rapidly rise of wafer temperature by the heating of the heating element, then simultaneously achieving uniform temperature distribution over the overall area of the wafer.
Furthermore, the present invention includes a ceramic heater for making up a wafer heating apparatus, which comprises a heating element on an insulating layer formed by forming a silicon oxide (Sio2) film having 0.05 to 2.0 xcexcm of thickness on said other major surface of a substrate made of silicon carbide ceramic and forming a glass insulating layer of 10 to 600 xcexcm thickness on the silicon oxide film as well as pad electrodes electrically connected to the heating element.
By controlling the flatness of the insulating layer to be 300 xcexcm or lower, the thickness of the heating element formed by a printing method can be controlled within a prescribed range and consequently, the precision of the temperature control of a wafer can be improved.